Pumps conveying the liquid coaxially through the driving electric motor are shown in U.S. Pat. No. 3,667,870. The disadvantage of these pumps is caused by the hollow shaft of the motor. Its diameter should be as small as possible to lower friction in the bushings and, on the other hand, as large as possible to form an unobstructed cross section for the liquid conveyed.
A second design of pumps, described in U.S. Pat. No. 3,846,050, is characterized by the magnetic gap between the stator and the rotor fulfilling two functions. It transfers energy between the driving and the driven part of the motor and acts at the same time as its bearing. The stator and the rotor are the sliding partners of said bearing. These pumps did not find any practical pumps the magnetic gap between stator and rotor follows a spherical surface. The rotating part is supported by a spherical step bearing so that shafts and bushings are not necessary.
To pivot a rotor on a single spherical step bearing requires continuous axial forces directed toward the step bearing. These forces are provided by the axial vector of the magnetic forces generated by the stator. All spherical pumps require this magnetic force.
A disadvantage of these pumps is the fact that the hydraulic thrust produced by the pressure generated by the impeller acts in opposition to the magnetic forces. When the electricity is switched off, the magnetic force of the electrical motor disappears immediately. In contrast, the hydraulic thrust, caused by the pressure generated by the rotating impeller, disappears slowly with decreasing rpm of the impeller. These remaining hydraulic forces cause the rotor to lift away from the step bearing after the motor is electrically switched off.
A further disadvantage of all spherical pumps is the tilt of the rotor-impeller-unit. The impeller is exposed to radial forces which are not equally distributed over the circumference of the impeller caused by the non-uniform pressure distribution within the spiral housing. Therefore, the degree of tilt depends upon the throttling of the discharged fluid. Any tilt reduces the mechanical as well as the electrical efficiency.
The main drawback is insufficient cooling of the windings. Spherical pumps have stators composed of pole pieces extending through the windings arranged in disc shaped layers. The pole pieces are relatively long in the axial direction. All waste heat of the windings is conducted to the pump housing through the limited cross section of the pole pieces. Therefore, the heat flux causes high winding temperatures. The surface conducting and dissipating the heat increases by the square of the linear dimension while the heat produced by the winding increases by the fourth power. This is the reason why spherical pumps are not suited for higher power levels.
Spherical pumps therefore found application only in the range of fractional horsepower performance.